Printers generally apply marking substances (e.g., toners) to receivers (e.g., paper). Toners generally include granules of wax or thermoplastic resin. These granules are applied image-wise to a receiver medium, then fixed to form a permanent image. In many printers, fixing is the step that determines the speed at which a printer can operate. It is therefore desirable to fix as quickly as possible to increase printer productivity. Electrophotographic printers are commonly used to form toner images on receiver media.
Various schemes have been described for fixing toners on a marked receiver. Some fixers pass the receiver through an oven. However, air has a low heat capacity, which limits its ability to transfer heat. Moreover, the hot air transfers heat not just to the toner, where the heat is desired, but also to the receiver. This failure to concentrate the applied heat can slow down the fixing process. It is also desirable to keep the temperature of paper receivers low, limiting the thermal power that can be applied.
Other schemes include irradiating the marked receiver (e.g., with infrared or microwave radiation). However, in order to avoid excessive heat absorption in the receiver, the frequency must be carefully chosen. Moreover, many receivers contain some water under normal conditions, as atmospheric moisture falls down its concentration gradient into dry porous or semi-porous sheets. Accordingly, it may not be possible to fix the toner without also heating the receiver.
Conventional fixing devices (sometimes called fusers or tackers) heat applied toner or press applied toner into the receiver. Some fixing devices heat indirectly, e.g., by irradiating the applied toner with infrared radiation. However, these devices can be slow. Moreover, contact fixers, e.g., those that pass marked receivers through a fixing nip with a heated roller, can boil or otherwise vaporize moisture in the receiver during fixing. These fixers generally use metal or polymer nip-forming rollers that substantially inhibit the resulting vapor from exiting the fixing area. This can result in blister formation in the receiver and other image defects. Furthermore, the heated roller on some fixers has a high thermal mass, making it more difficult to change the roller temperature to adjust for variations in fixing characteristics between pages.
U.S. Pat. No. 4,943,816 to Sporer, entitled “High quality thermal jet printer configuration suitable for producing color images,” discloses the use of a marking fluid containing no dye so that a latent image in the form of fluid drops is formed on a piece of paper. The marking fluid is relatively non-wetting to the paper. Sporer teaches the use of a 300 dpi thermal inkjet printer to produce the latent image. Surface tension then causes colored powder to adhere to the fluid drops. Sporer teaches that only that portion of the droplet that has not penetrated or feathered into the paper is available for attracting dry ink, so this process is unsuitable for highly-absorbent papers such as newsprint. It is desirable to be able to tone and fix on a wide range of receiver types. Moreover, Sporer's process does not remove moisture from the receiver, so blistering can still result. Also, this process is a hybrid of inkjet and powder printing, so is not suitable for use in conventional electrophotographic printers.
There is, therefore, a continuing need for ways of fixing toner on receivers, e.g., to permit producing high-quality images at high speed using electrophotographic printers.